The invention is directed to a combined service brake and spring type brake cylinder having a diaphragm, which may be impinged with pressure, situated in a housing of the service brake cylinder, which delimits a service brake chamber, which may be impinged with a service brake pressure, on one side and a chamber receiving a return spring on the other side. A spring type brake piston, which is situated in a housing of the spring type brake cylinder and is actuatable by a storage spring, delimits a spring type brake chamber on one side and a spring chamber, which receives a storage spring, on the other side, and has a piston rod, which carries a ventilation valve that produces or blocks a flow connection between the spring chamber and the service brake chamber. The ventilation valve contains a piston working together with a valve seat, which is loaded by the pressure in the spring chamber in the open position and releases a flow cross-section in its position lifted off of the valve seat.
Such a combined service brake and spring type brake cylinder is known, for example, from DE 40 11 739 A1. The ventilation valve is situated therein on its end in the hollow piston tube of the spring type brake piston, which may project into the service brake chamber as a function of the operating state. The ventilation valve has the object, upon disengagement of the parking brake, of dissipating the overpressure arising due to the return of the parking brake piston and thus the decreasing volume of the spring chamber, in that it is switched thereby into its open position and produces a flow connection between the spring chamber and the service brake chamber. When starting on a level roadway, the service brake chamber is ventilated and is connected to a ventilator of a pressure regulator module, because service braking is not necessary after the parking brake is disengaged. At least a part of the excess air volume in the spring chamber may then flow out, which thus does not reach the atmosphere directly therefrom, e.g., via a valve situated in the wall of the spring chamber, but rather via the aeration and ventilation path of the service brake chamber. Therefore, one also refers to internal ventilation in this context.
However, when starting on a rising roadway, it is necessary to at least briefly additionally apply the service brake after or during the disengagement of the parking brake to prevent the vehicle from rolling backward. In this case, the service brake chamber is aerated. In the event of sufficiently high brake demand by the driver, the service brake pressure existing in the service brake chamber and also on one side of the piston may be capable of keeping it closed against the effect of the pressure building up in the spring chamber on the valve seat and thus the ventilation valve. However, if the service brake pressure and/or the service brake pressure gradient are below certain threshold values because of a correspondingly low service brake demand of the driver, the service brake pressure existing on one side of the piston is insufficient to keep the ventilation valve closed. Compressed air then flows from the service brake chamber via the open ventilation valve into the spring chamber. It escapes from there via the piston seal and the housing seal into the atmosphere, which on one hand causes disturbing noises. On the other hand, the air volume flowing out via the ventilation valve is no longer available for dissipating the service brake force.
The present invention is based on the object of refining a combined service brake and spring type brake cylinder of the type cited at the beginning in such a way that the above-mentioned disadvantages are avoided.
The invention is based on the idea that the ventilation valve contains at least one further piston, which works together with a further valve seat and is loaded by the pressure in the spring chamber in the open position and by the pressure in the service brake chamber in the closed position, which releases at least one further flow cross-section in its position lifted off of the further valve seat, the one piston being lifted off of the valve seat and the further piston being sealed against the further valve seat when the parking brake is applied and the service brake is actuated simultaneously using service brake pressures which are less than a pressure threshold, and/or using service brake pressure gradients which are less than a pressure gradient threshold. Because of the smaller flow cross-section, the volume flow or the air volume, which flows out from the service brake chamber via the partially open ventilation valve into the spring chamber and therefrom via the seals to the outside, is less than in the prior art, which has an advantageous effect on the noise behavior and the air consumption.
Advantageous refinements and improvements of the invention specified in the independent claims are described herein.
More detailed information is disclosed in the following description of exemplary embodiments.